The huntingtin gene, also referred to as the HTT or HD (Huntington's disease) gene, encodes for the huntingtin protein. The huntingtin gene is a large gene of about 13.5 kb (huntingtin protein is about 350 kDa). Huntington's disease is a genetic neurodegenerative disorder caused by a genetic mutation in the huntingtin gene. The genetic mutation involves a DNA segment of the huntingtin gene known as the CAG trinucleotide repeat. Normally, the CAG segment in the huntingtin gene of humans is repeated multiple times, i.e. about 10-35 times. People that develop Huntington's disease have an expansion of the number of CAG repeats in at least one allele. An affected person usually inherits the mutated allele from one affected parent. In rare cases, an individual with Huntington's disease does not have a parent with the disorder (sporadic HD). People with 36 to 39 CAG repeats may develop signs and symptoms of Huntington disease, while people with 40 or more repeats almost always develop the disorder. The increase in the size of the CAG repeat leads to the production of an elongated (mutated) huntingtin protein. This protein is processed in the cell into smaller fragments that are cytotoxic and that accumulate and aggregate in neurons. This results in the disruption of normal function and eventual death of neurons. This is the process that occurs in the brain which underlies the signs and symptoms of Huntington's disease.
RNA interference (RNAi) is a naturally occurring mechanism that involves sequence specific down regulation of mRNA. The down regulation of mRNA results in a reduction of the amount of protein that is expressed. RNA interference is triggered by double stranded RNA. One of the strands of the double stranded RNA is substantially or completely complementary to its target, the mRNA. This strand is termed the guide strand. The mechanism of RNA interference involves the incorporation of the guide strand in the RNA-induced silencing complex (RISC). This complex is a multiple turnover complex that via complementary base paring binds to its target mRNA. Once bound to its target mRNA it can either cleave the mRNA or reduce translation efficiency. RNA interference has since its discovery been widely used to knock down specific target genes. The triggers for inducing RNA interference that have been employed involve the use of siRNAs or shRNAs. In addition, molecules that can naturally trigger RNAi, the so called miRNAs, have been used to make artificial miRNAs that mimic their naturally occurring counterparts. These strategies have in common that they provide for substantially double stranded RNA molecules that are designed to target a gene of choice. RNAi based therapeutic approaches that utilise the sequence specific modality of RNAi are under development and several are currently in clinical trials (see i.a. Davidson and McCray, Nature Reviews—Genetics, 2011; Vol. 12; 329-340).
As Huntington's disease involves the expression of a mutant huntingtin protein, the accumulation thereof leading to disease, RNA interference provides for an opportunity to treat the disease as it can reduce expression of the huntingtin gene. The paradigm underlying this approach involves a reduction of the mutant Htt protein to thereby reduce the toxic effects resulting from the mutant Htt protein to achieve a reduction and/or delay of Huntington's disease symptoms, or even to prevent Huntington's disease symptoms altogether. Targeting huntingtin gene suppression has been hypothesized in the prior art, including the listing of about two thousand of hypothetical siRNA target sequences (WO2005105995). Strategies to reduce huntingtin gene expression are known in the art and involve the specific targeting of mutant huntingtin genes (e.g. US20090186410, US20110172291). Alternatively, RNA interference has also been employed to target both mutant and non-mutant genes (e.g. Rodriguez-Lebron et al., 2005, Mol Ther. Vol 12 No. 4: 618-633; Franich et al., 2008, Mol Ther, Vol. 16 No. 5; 947-956; Drouet et al., 2009, Annals of Neurology; Vol. 65 No. 3; 276-285 and McBride et al. Mol Ther. 2011 December; 19(12):2152-62; US20080015158, WO2008134646). In the latter case, knockdown of the wild type Huntingtin protein was shown not to have any apparent detrimental effects.